The leishmaniases are a group of morphologically-similar flagellates that are transmitted by sandfly vector, survive within the phagocytes of the vertebrate host and induce a spectrum of diseases of varying severity. Leishmaniasis is more feared for its extreme tissue morbidity, its frequent complications, and the inconvience and discomfort of its treatment, rather than its levels of fatality. The mainline drugs against the parasite are pentavalent antimonial compounds that often require hospitalization during administration, and have complications due to toxicity. There is therefore, an urgent need for alternative drug therapies for these infections. This Project proposes two mutually-supportive goals designed to produce a new chemotherapy for leishmaniasis. The amastigote form of Leishmania resides within the macrophage, in an acidic, hydrolytically-active lysosomal compartment. This compartment may be accessed through the endocytic network of the host cell. The first goal of this proposal is to exploit the macrophage-specific nature of some of the endocytic receptors on these phagocytes to facilitate selective delivery and concentration of active compounds within the macrophage. As the macrophage is known to play a role in clearance of drugs from the blood, one should be able to enhance this behavior by complexing the drugs with ligands for the mannose/fucose receptor, which is a macrophage-specific lectin. The efficiency of this strategy will be evaluated with ligands coupled to reporter molecules such as biotin or dinitrophenol. Delivery to the parasitophorous vacuole will be examined by immunoelectron microscopy. The second goal of the Project involves the exploitation of a demonstrated selectivity that the cysteine proteinases of the Leishmania amastigote have shown for certain peptidic substrates that are resistant to hydrolysis by the cathepsins of the macrophage's lysosome. Using isolated phagosomes, we have shown that the parasite enzymes exhibit a markedly different cleavage site recognition, and can be inhibited by peptidoketones, which block parasite growth. We intend to pursue these studies in the development of parasite-specific cysteine proteinase inhibitors. These inhibitors will be complexed with macrophage-specific ligands, identified in the first goal, to produce an antileishmanial compound that will be delivered preferentially to macrophages.